Pseudomonas aeruginosa (PA) is a bacterium that poses an immediate threat to public health because of its leading role in causing nosocomial infections, emergence of multidrug-resistance, virulence potential, and the lack of new drugs targeting this pathogen in the development pipeline. During acute infections, PA utilizes the type III secretion system (TTSS) to deliver cytotoxins (i.e. ExoU, ExoS) into host cells upon contact, ultimately allowing PA to cross epithelial barrier and spread systemically or cause cell death. Most PA isolates contain either exoS or exoU gene;its presence varies by clinical sites or disease backgrounds. Excess morbidity and mortality have been demonstrated for acute pneumonia caused by ExoU-secreting PA and FQ-resistant PA, raising the possibility that antibiotic resistance may be linked with enhanced virulence. Preliminary data indicates that FQ-resistant PA population is predominated by exoU+ strains, with increased cytotoxicity towards cultured epithelial cells for those showing higher degree of resistance. We hypothesize that exoU+ PA strains adapt more readily to FQ exposure by developing mutations at target enzymes (e.g. DNA gyrase) which simultaneously confer resistance to FQ and upregulate TTSS gene transcription due to changes in DNA supercoiling. We are first to propose a systematic approach to determine if FQ-resistant PA strains are more virulent as a necessary initial step towards the development of novel management strategies urgently needed for infections caused by multi-drug resistant PA. We will examine a large sample of respiratory isolates (n=216) obtained from patients with PA pneumonia and available outcome data already collected to: 1) determine if FQ resistance is more frequently observed in exoU+ compared to exoS+ strains, 2) assess the extent to which co-expression of resistance and virulence in clinical isolates translates into a biologic effect in in vitro cellular model and correlates with outcomes of patients, and 3) study the dynamics of virulence expression as resistance mutations accumulate in PA strains following repeated exposure to FQ. If we prove that PA evolves into a "superbug" in response to FQ exposure by becoming more resistant and more virulent, we will have a strong basis to 1) prescribe more aggressive treatment with early and more intensive therapy, 2) severely limit indiscriminant prescribing of the FQs and at the same time, intensify current infection control efforts to minimize spread of FQ-resistant and virulent strains. We will have a strong basis to screen existing compounds that can block TTSS virulence as well as identify novel drug targets based on a better understanding of the molecular pathways that regulate resistance and virulence expression. Our hypothesis if proven will help advance the development of immunotherapy against the type III effectors (pcrV vaccine, anti-PcrV antibody) and also rapid diagnostics to determine virulence potential as an essential part of patient care. PUBLIC HEALTH RELEVANCE: The goal of this project is to confirm if Pseudomonas aeruginosa, a leading cause of nosocomial infections, becomes more virulent as it becomes resistant to the fluoroquinolone antibiotics and prove the molecular model by which this co-expression occurs. The lack of new drugs targeting this pathogen in the development pipeline underscores the urgent need to perform this study as a necessary initial step in order to gain a better understanding of the operative virulence strategies in the resistant strains so that novel therapeutics aim at disarming virulence can be developed.